Astronomers Watch a Black Hole Eat a Meal

Scientists have pieced together the journey of a bundle of doomed matter as it orbited a black hole four times, an observational first. Their technique provides a new method to measure the mass of a black hole; and this may enable the testing of Einstein’s theory of gravity to a degree few thought possible.

A team led by Dr. Kazushi Iwasawa at the Institute of Astronomy (IoA) in Cambridge, England, followed the trail of hot gas over the course of a day as it whipped around the supermassive black hole roughly at the same distance the Earth orbits the Sun. Quickened by the extreme gravity of the black hole, however, the orbit took about a quarter of a day instead of a year.

The scientists could calculate the mass of the black hole by plugging in the measurements for the energy of the light, its distance from the black hole, and the time it took to orbit the black hole — a marriage of Einstein’s general relativity and good old-fashioned Keplerian physics.

Iwasawa and his colleague at the IoA, Dr. Giovanni Miniutti, present this result today during a Web-based press conference in New Orleans at the meeting of the High Energy Astrophysics Division of the American Astronomical Society. Dr. Andrew Fabian of the IoA joins them on an article appearing in an upcoming issue of the Monthly Notices of the Royal Astronomical Society. The data is from the European Space Agency’s XMM-Newton observatory.

The team studied a galaxy named NGC 3516, about 100 million light years away in the constellation Ursa Major, home to the Big Dipper (or, the Plough). This galaxy is thought to harbour a supermassive black hole in its core. Gas in this central region glows in X-ray radiation as it is heated to millions of degrees under the force of the black hole’s gravity.

XMM-Newton captured spectral features from light around the black hole, displayed on a spectrograph with spikes indicating certain energy levels, similar in appearance to the jagged lines of a cardiograph. During the daylong observation, XMM captured a flare from excited gas orbiting the black hole as it whipped around four times. This was the crucial bit of information needed to measure the black hole mass.

The scientists already knew the distance of the gas from the black hole from its spectral feature. (The extent of gravitational redshift, or energy drain revealed by the spectral line, is related to how close an object is to a black hole.) With an orbital time and distance, the scientists could pin down a mass measurement — between 10 million and 50 million solar masses, in agreement with values obtained with other techniques.

While the calculation is straightforward, the analysis to understand the orbital period of an X-ray flare is new and intricate. Essentially, the scientists detected a cycle repeated four times: a modulation in the light’s intensity accompanied by an oscillation in the light’s energy. The energy and cycle observed fit the profile of light gravitationally redshifted (gravity stealing energy) and Doppler shifted (a gain and loss in energy as orbiting matter moves towards and away from us).

The analysis technique implies, to this science team’s surprise, that the current generation of X-ray observatories can make significant gains in measuring black hole mass, albeit with long observations and black hole systems with long-lasting flares. Building upon this information, proposed missions such as Constellation-X or XEUS can make deeper inroads to testing Einstein’s math in the laboratory of extreme gravity.